Why Do Cutting Tool Charts Give Recommended Parameters in a Broad Range?
The answer has to do with the differences from cut to cut, as well as the differences that make some shops more capable than others.
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Tom Delio is one of the nation's leading authorities on high speed machining and machine tool dynamics. He answers questions from readers on challenges related to high speed machining. To ask your question, visit our High Speed Machining Zone and click on “Ask an Expert.”
When I use the recommended rpm for steel machining from a cutting tool chart, I frequently have to adjust speed and feed. The chart doesn’t even give exact speed and feed rate values. Why?
Response from Tom Delio, co-founder of BlueSwarf
The cutting tool charts are usually based on two types of limits. They are (1) a stress limit for the cutting edge (strictly tied to the chip load) and (2) a thermal limit for the maximum surface or rotational velocity of the cutter (strictly tied to the rpm). The combination of rpm and chip load then dictates the feed rate.
Cutting tool companies either provide a broad range for these two primary values (chip load and surface velocity) or just a starting value. They do this because they test their tooling under generally favorable conditions (stiff cutters, short stickout, solid machines, quality materials) for the various materials. There are numerous grades of materials as well as heat treats—too many for the cutting tool companies to effectively cover. So they cannot give an exact spindle speed or chip load, just rough starting values.
A final variable is the vibration condition of the cut. This is dependent on many factors that are again outside the realm of the cutting tool companies, as they have no control over how you might apply their product beyond the general guidelines. For example, you might put the cutter in an extended long reach holder, or you might try to machine a very flexible structure. Also, you might machine using a high or low radial engagement. Trying to cover all of these factors would make a cutting tool chart unmanageable and unreadable.
This is where the skill and experience of a job shop pays off, and distinguishes one shop from another in terms of making profitable parts. My best advice is to do some systematic tests or look for added sources of information and develop a consistent approach to your machining—recording the acceptable operating parameters when you settle on them. Beyond that, use consistent tool practices, tool setups and tool types.